Uninterruptible power supply system

ABSTRACT

An uninterruptible power supply system, including: a plurality of uninterruptible power supply devices which are connected in parallel with respect to a load, and switch between power supplies supplying power to the load depending on states of the power supplies; a control unit controlling an operation of switching between the power supplies by the uninterruptible power supply devices; and a storage battery connected to the plurality of uninterruptible power supply devices in common. The uninterruptible power supply device has a converter converting AC power of an AC power supply into DC power, a contactor switching between the DC power converted by the converter and DC power input from the storage battery, and an inverter inverting the DC power into AC power and supplying the power to the load. The control unit deactivates the converter which does not contribute to supplying a power amount required for the load, of a plurality of converters.

TECHNICAL FIELD

The present invention relates to an uninterruptible power supply systemincluding a plurality of uninterruptible power supply devices connectedin parallel, each uninterruptible power supply device convertingalternating current (AC) power into direct current (DC) power andinverting the converted DC power or DC power of a storage battery intoAC power to supply the power to a load.

BACKGROUND ART

An uninterruptible power supply system disclosed in Japanese PatentLaying-Open No. 2011-72068 (Patent Document 1) includes a plurality ofuninterruptible power supply devices, a switching circuit switching to abypass power supply with outage in case of failure of eachuninterruptible power supply device, and a switch opening/closing anoutput of the switching circuit. The uninterruptible power supply systemdisclosed in Patent Document 1 is configured such that connectionbetween the switching circuit and a load can be selected by the switchto select a configuration in which one uninterruptible power supplydevice is connected, or a configuration in which the plurality ofuninterruptible power supply devices are connected in parallel.

Thus, even when one of the uninterruptible power supply devices fails,the uninterruptible power supply system disclosed in Patent Document 1can supply power in a short time, and thus can lower the risk ofdeactivation of the load due to an outage of the bypass power supplywithin a time required to repair the uninterruptible power supplydevice.

CITATION LIST Patent Document

PTD 1: Japanese Patent Laying-Open No. 2011-72068

SUMMARY OF INVENTION Technical Problem

Since improved reliability is required in conventional uninterruptiblepower supply systems, a plurality of uninterruptible power supplydevices are connected in parallel, and operated in parallel by a commonstorage battery. Further, conventional uninterruptible power supplysystems activate all of the uninterruptible power supply devices,irrespective of a power amount required for a load (actual poweramount), if a total power amount of the uninterruptible power supplydevices is smaller than a total rated power amount. Thus, conventionaluninterruptible power supply systems have had a problem that efficiencyof utilizing the plurality of uninterruptible power supply devicesconnected in parallel is poor.

Further, an uninterruptible power supply device includes a converter(conversion unit) converting AC power into DC power, and an inverter(inversion unit) inverting DC power into AC power, and as long as theuninterruptible power supply device is activated, power losses occur insemiconductor elements constituting the converter and the inverter.Thus, conventional uninterruptible power supply systems have had aproblem that power consumption cannot be reduced due to power losses inuninterruptible power supply devices.

Furthermore, in conventional uninterruptible power supply systems, whenany of the plurality of uninterruptible power supply devices connectedin parallel is deactivated, all circuits including the converter and theinverter constituting the deactivated uninterruptible power supplydevice are deactivated. Thus, in conventional uninterruptible powersupply systems, when the deactivated uninterruptible power supply deviceis reactivated to supply the power amount required for the load, it isnecessary to activate all circuits in turn, which results in an increasein the time taken to activate the deactivated uninterruptible powersupply device.

Accordingly, the present invention has been made to solve theaforementioned problems, and one object of the present invention is toprovide an uninterruptible power supply system having a high efficiencyof utilizing a plurality of uninterruptible power supply devicesconnected in parallel, and capable of reducing power consumption.

Solution to Problem

In order to solve the aforementioned problems, the present invention isdirected to an uninterruptible power supply system, including: aplurality of uninterruptible power supply devices which are connected inparallel with respect to a load, and switch between power suppliessupplying power to the load depending on states of the power supplies; acontrol unit controlling an operation of switching between the powersupplies by the uninterruptible power supply devices; and a storagebattery connected to the plurality of uninterruptible power supplydevices in common. The uninterruptible power supply device has aconversion unit converting AC power of an AC power supply into DC power,a switching unit switching between the DC power converted by theconversion unit and DC power input from the storage battery, and aninversion unit inverting the DC power converted by the conversion unitor the DC power input from the storage battery into AC power andsupplying the power to the load. The control unit deactivates theconversion unit which does not contribute to supplying a power amountrequired for the load, of a plurality of conversion units.

Advantages Effects of Invention

According to the uninterruptible power supply system in accordance withthe present invention, in the case where the power amount required forthe load is smaller than a total rated power amount output by theactivated uninterruptible power supply devices (conversion units) (i.e.,in the case of a low load), the control unit deactivates the conversionunit which does not contribute to supplying the power amount requiredfor the load, of the plurality of conversion units. Therefore,efficiency of utilizing the plurality of uninterruptible power supplydevices connected in parallel can be enhanced. Further, since theconversion unit which does not contribute to supplying the power amountrequired for the load is deactivated in the uninterruptible power supplysystem in accordance with the present invention, power losses in thedeactivated conversion unit are suppressed, and thus power consumptioncan be reduced. Furthermore, since only the conversion unit isdeactivated in the uninterruptible power supply system in accordancewith the present invention, the time taken to activate a deactivateduninterruptible power supply device can be shortened when compared witha case where all circuits in an uninterruptible power supply device aredeactivated.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view showing a configuration of an uninterruptiblepower supply system in accordance with Embodiment 1 of the presentinvention.

FIG. 2 is a circuit diagram showing a configuration of a converter ofthe uninterruptible power supply system in accordance with Embodiment 1of the present invention.

FIG. 3 is a timing chart for illustrating a switching loss in an IGBT.

FIG. 4 is a flowchart for illustrating an operation of theuninterruptible power supply system in accordance with Embodiment 1 ofthe present invention.

FIG. 5 is a flowchart for illustrating an operation of anuninterruptible power supply system in accordance with Embodiment 2 ofthe present invention.

FIG. 6 is a flowchart for illustrating an operation of anuninterruptible power supply system in accordance with Embodiment 3 ofthe present invention.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments in accordance with the present invention willbe described with reference to the drawings.

Embodiment 1

FIG. 1 is a schematic view showing a configuration of an uninterruptiblepower supply system in accordance with Embodiment 1 of the presentinvention. An uninterruptible power supply system 100 shown in FIG. 1includes an AC input unit 1, an input/output switching unit 2, a controlunit 3, a DC branch unit 4, a storage battery 5, an output unit 6, ACoutput units 7, 8, and uninterruptible power supply devices 10, 20, 30.

AC input unit 1 is connected to an AC power supply not shown to supplypower to uninterruptible power supply devices 10, 20, 30. The AC powersupply connected to AC input unit 1 is an AC power supply such as acommercial power supply or a private power generator.

AC output units 7, 8 are connected to a load not shown (such as acomputer or communication equipment) to supply power fromuninterruptible power supply devices 10, 20, 30 to the load. If thepower to be supplied from uninterruptible power supply devices 10, 20,30 is, for example, three-phase AC power, AC output units 7, 8 areconnected to the load using three-phase three-wire distribution lines.

Input/output switching unit 2 switches connections between AC input unit1 and uninterruptible power supply devices 10, 20, 30 and connectionsbetween uninterruptible power supply devices 10, 20, 30 and AC outputunits 7, 8 to supply power to the load. It is noted that input/outputswitching unit 2 includes a maintenance bypass line connecting AC inputunit 1 to AC output units 7, 8 when a maintenance work is performed onuninterruptible power supply devices 10, 20, 30. The maintenance bypassline includes a breaker 71, a transformer 72, a switch 73, a thyristorswitch 74, and a contactor 75. Breaker 71 is a switch preventing highpower from suddenly flowing into the maintenance bypass line.Transformer 72 is an insulation transformer transforming the AC voltageof AC input unit 1. Switch 73 is a switch connecting transformer 72 tothyristor switch 74. Thyristor switch 74 is a semiconductor switchcapable of switching output power of uninterruptible power supplydevices 10, 20, 30 faster than contactors 61 to 63 in output unit 6.Contactor 75 is a switch for outputting AC power of AC input unit 1 toAC output units 7, 8 through the maintenance bypass line.

Input/output switching unit 2 includes a breaker 11 for preventing asudden inflow of high power, between AC input unit 1 and uninterruptiblepower supply device 10. Similarly, input/output switching unit 2includes a breaker 21 between AC input unit 1 and uninterruptible powersupply device 20, and includes a breaker 31 between AC input unit 1 anduninterruptible power supply device 30.

Input/output switching unit 2 further includes a transformer 12transforming the AC voltage of AC input unit 1, between AC input unit 1and uninterruptible power supply device 10. Similarly, input/outputswitching unit 2 includes a transformer 22 between AC input unit 1 anduninterruptible power supply device 20, and includes a transformer 32between AC input unit 1 and uninterruptible power supply device 30.

Furthermore, input/output switching unit 2 includes a switch 76connecting the maintenance bypass line to AC output units 7, 8, andswitches 77 to 79 connecting uninterruptible power supply devices 10,20, 30 to AC output units 7, 8.

Uninterruptible power supply device 10 includes a contactor 13, an ACreactor 14, a converter 15, an electrolytic capacitor 16, an inverter17, a transformer 18, and a capacitor 19. Contactor 13 is a switch forinputting the AC power having the voltage transformed by transformer 12to uninterruptible power supply device 10. AC reactor 14 is a filter forshaping the waveform of the AC power input to uninterruptible powersupply device 10. Converter 15 is a conversion unit converting the ACpower having the waveform shaped by AC reactor 14 into DC power.

FIG. 2 is a circuit diagram showing a configuration of converter 15 ofuninterruptible power supply system 100 in accordance with Embodiment 1of the present invention. Converter 15 shown in FIG. 2 includes threeIGBTs (Insulated Gate Bipolar Transistors) 151 connected in parallelbetween a positive side of a power supply and a load, three IGBTs 152connected in parallel between a negative side of the power supply andthe load, and FWDs (Free Wheeling Diodes) 153 connected in parallel withIGBTs 151, 152.

Converter 15 can convert AC power into DC power by driving each IGBT151, 152 at an appropriate time. Losses caused by driving IGBT 151, 152include a steady loss caused by passing a current through IGBT 151, 152,and a switching loss caused by switching IGBT 151, 152.

FIG. 3 is a timing chart for illustrating a switching loss in IGBT 151,152. The waveforms shown in FIG. 3 indicate a waveform of a voltage Vapplied to a gate electrode of IGBT 151, 152, and a waveform of acurrent I flowing between a collector electrode and an emitter electrodeof IGBT 151, 152. IGBT 151, 152 enters an ON state at the time whenvoltage V applied to the gate electrode falls, and current I startsflowing between the collector electrode and the emitter electrode. If atime taken from when voltage V applied to the gate electrode falls towhen current I flowing between the collector electrode and the emitterelectrode enters a steady state is referred to as a switching time t, aswitching loss P_(sw) in IGBT 151, 152 is calculated by multiplyingswitching time t by power (voltage V×current I) consumed in IGBT 151,152, i.e., switching loss P_(sw)=voltage V×current I×switching time t.Accordingly, in the case where a power amount required for the load issmaller than a total rated power amount output by a plurality ofactivated converters (i.e., in the case of a low load), even if there isa converter which does not contribute to supplying the power amountrequired for the load, switching loss P_(sw) is caused every time whenthe IGBT is switched, unless the converter is deactivated.

Since converter 15 uses IGBTs 151, 152, it has a fast switching time tof 0.25 μm. However, converter 15 is not limited to use IGBTs 151, 152as long as it uses semiconductor switches, and may use bipolartransistors, thyristors, or the like. When converter 15 uses bipolartransistors, it has switching time t of 2.5 μm, which is slower thanthat when it uses the IGBTs.

Further, the reason why the waveform of voltage V shown in FIG. 3 doesnot become 0 V even when it falls is because a steady loss P_(c) iscaused in IGBT 151, 152. Accordingly, in the case where the power amountrequired for the load is smaller than the total rated power amountoutput by the plurality of activated converters (i.e., in the case of alow load), even if there is a converter which does not contribute tosupplying the power amount required for the load, steady loss P_(c) iscaused in the IGBT unless the converter is deactivated.

Thus, in uninterruptible power supply system 100 in accordance withEmbodiment 1 of the present invention, in the case where the poweramount required for the load is small (i.e., in the case of a low load),the converter which does not contribute to supplying the power amountrequired for the load is deactivated, and thereby switching loss P_(sw)and steady loss P_(c) are suppressed and power consumption is reduced,as described later.

Referring back to FIG. 1, electrolytic capacitor 16 is a smoothingcapacitor for smoothing the DC power converted by converter 15. Inverter17 is an inversion unit inverting the DC power smoothed by electrolyticcapacitor 16 or DC power input from storage battery 5 into AC power.Converter 15 and inverter 17 have the same circuit configuration, sincethey are merely different in that the former converts AC power into DCpower and the latter inverts DC power into AC power. Thus, inverter 17also has a configuration including three IGBTs 151 connected in parallelbetween a positive side of a power supply and a load, three IGBTs 152connected in parallel between a negative side of the power supply andthe load, and FWDs 153 connected in parallel with IGBTs 151, 152, asshown in FIG. 2.

Transformer 18 transforms the voltage of the AC power inverted byinverter 17. Capacitor 19 is a filter for shaping the waveform of the ACpower having the voltage transformed by transformer 18. The AC powerhaving the waveform shaped by capacitor 19 is output fromuninterruptible power supply device 10. The AC power output fromuninterruptible power supply device 10 is output from AC output units 7,8, via contactor 61 of output unit 6.

Output unit 6 includes contactors 61 to 63. Output unit 6 switchescontactors 61 to 63 to connect uninterruptible power supply devices 10,20, 30 to AC output units 7, 8 and supply the AC power output fromuninterruptible power supply devices 10, 20, 30 to the load.

Uninterruptible power supply device 20 includes a contactor 23, an ACreactor 24, a converter 25, an electrolytic capacitor 26, an inverter27, a transformer 28, and a capacitor 29. Uninterruptible power supplydevice 30 includes a contactor 33, an AC reactor 34, a converter 35, anelectrolytic capacitor 36, an inverter 37, a transformer 38, and acapacitor 39. Since the configurations of uninterruptible power supplydevices 20, 30 are the same as that of uninterruptible power supplydevice 10, a detailed description thereof will not be repeated.

Control unit 3 includes a converter control unit 300, converteroperation command circuits 301 to 303, inverter operation commandcircuits 304 to 306, and signal generation circuits 307 to 309. It isnoted that control unit 3 also controls an operation of switchingbetween power supplies (the AC power supply connected to AC input unit 1and the DC power supply of the storage battery) for uninterruptiblepower supply devices 10, 20, 30.

Converter control unit 300 receives a feedback value of a converter loadcurrent from each converter 15, 25, 35 and adds the received feedbackvalue to calculate a total power amount output by converters 15, 25, 35for supplying a power amount required for the load. Further, convertercontrol unit 300 compares the calculated total power amount with a totalrated power amount output, when at least one of activated converters 15,25, 35 is deactivated, by the remaining converters, and outputs signalsfor commanding to select the number of converters 15, 25, 35 toconverter operation command circuits 301 to 303 for converters 15, 25,35.

Signal generation circuits 307 to 309 receive instructions to activateand deactivate converters 15, 25, 35 and inverters 17, 27, 37 from aninput unit not shown, and output instruction signals to converteroperation command circuits 301 to 303 and inverter operation commandcircuits 304 to 306.

Converter operation command circuits 301 to 303 output command signalsfor commanding activation and deactivation to converters 15, 25, 35,based on the number selection command signals output by convertercontrol unit 300 and the instruction signals output by signal generationcircuits 307 to 309.

Inverter operation command circuits 304 to 306 output command signalsfor commanding activation and deactivation to inverters 17, 27, 37,based on the instruction signals output by signal generation circuits307 to 309.

DC branch unit 4 includes breakers 44 to 48. Breakers 44, 45, 46 areswitches preventing high power from suddenly flowing intouninterruptible power supply devices 10, 20, 30. Breakers 47, 48 areswitches preventing high power from suddenly flowing into storagebattery 5. It is noted that uninterruptible power supply devices 10, 20,30 include contactors 41, 42, 43. Contactors 41, 42, 43 are switches forconnecting uninterruptible power supply devices 10, 20, 30 to storagebattery 5, and are switching units switching between the DC powerconverted by converters 15, 25, 35 and the DC power input from storagebattery 5.

Storage battery 5 is connected to uninterruptible power supply devices10, 20, 30 in common. Storage battery 5 is composed of two batteries 51,52. It is noted that storage battery 5 is not limited to be composed oftwo batteries 51, 52, and may be composed of a single battery or threeor more batteries.

Next, an operation of uninterruptible power supply system 100 inaccordance with Embodiment 1 of the present invention will be described.FIG. 4 is a flowchart for illustrating the operation of uninterruptiblepower supply system 100 in accordance with Embodiment 1 of the presentinvention. First, converter control unit 300 activates converters 15,25, 35 of all uninterruptible power supply devices 10, 20, 30 (stepS41). Specifically, converter control unit 300 outputs number selectioncommand signals for selecting all converters 15, 25, 35 to converteroperation command circuits 301 to 303 for converters 15, 25, 35, andsignal generation circuits 307 to 309 output instruction signals foractivating converters 15, 25, 35 to converter operation command circuits301 to 303.

Converter operation command circuits 301 to 303 output command signalsfor commanding activation to converters 15, 25, 35, based on the numberselection command signals output by converter control unit 300 and theinstruction signals output by signal generation circuits 307 to 309.Converters 15, 25, 35 are activated based on the command signals outputby converter operation command circuits 301 to 303.

Subsequently, converter control unit 300 compares the total power amountoutput by converters 15, 25, 35 for supplying the power amount requiredfor the load, with the total rated power amount output, when at leastone of activated converters 15, 25, 35 is deactivated, by the remainingconverters, and determines whether or not the total power amount issmaller than the total rated power amount (step S42).

When converter control unit 300 determines that the total power amountis smaller than the total rated power amount (YES in step S42),converter control unit 300 deactivates at least one of activatedconverters 15, 25, 35 (step S43). For example, when a rated power amountoutput by each converter 15, 25, 35 is 1 kWh, converter control unit 300determines that, if the total power amount is 1.8 kWh, it is smallerthan a total rated power amount output, when one of activated converters15, 25, 35 is deactivated, by the remaining converters (1 kWh×2 units=2kWh).

That is, if the total power amount is 1.8 kWh, even when convertercontrol unit 300 deactivates one converter 35, the total power amount issmaller than the total rated power amount output by activated converters15, 25 (1 kWh×2 units=2 kWh). Thus, converter control unit 300deactivates converter 35 by outputting a number selection command signal“OFF” to converter operation command circuit 303 for converter 35.

When converter control unit 300 determines that the total power amountis not less than the total rated power amount (NO in step S42),converter control unit 300 returns the processing to step S42 to monitora change in the total power amount. Further, after converter controlunit 300 deactivates at least one of activated converters 15, 25, 35(step S43), converter control unit 300 returns the processing to stepS42 to monitor a change in the total power amount.

It is noted that converter control unit 300 may deactivate one ofconverters 15, 25, 35, or two or more of converters 15, 25, 35. Whenconverter control unit 300 deactivates one of converters 15, 25, 35,converter control unit 300 compares the total power amount with thetotal rated power amount output, when one of activated converters 15,25, 35 is deactivated, by the remaining converters. When convertercontrol unit 300 deactivates two of converters 15, 25, 35, convertercontrol unit 300 compares the total power amount with a total ratedpower amount output, when two of activated converters 15, 25, 35 aredeactivated, by the remaining converter.

As described above, according to uninterruptible power supply system 100in accordance with Embodiment 1 of the present invention, in the casewhere the power amount required for the load is smaller than a totalrated power amount output by activated converters 15, 25, 35 (i.e., inthe case of a low load), converter control unit 300 deactivates theconverter which does not contribute to supplying the power amountrequired for the load, of converters 15, 25, 35. Therefore, efficiencyof utilizing the plurality of uninterruptible power supply devices 10,20, 30 connected in parallel can be enhanced. Further, since converter15, 25, 35 which does not contribute to supplying the power amountrequired for the load is deactivated in uninterruptible power supplysystem 100 in accordance with Embodiment 1 of the present invention,switching loss P_(sw) and steady loss P_(c) in the deactivated converterare suppressed, and thus power consumption can be reduced. Furthermore,since only converter 15, 25, 35 is deactivated in uninterruptible powersupply system 100 in accordance with Embodiment 1 of the presentinvention, the time taken to activate deactivated uninterruptible powersupply device 10, 20, 30 can be shortened when compared with a casewhere all circuits in uninterruptible power supply device 10, 20, 30 aredeactivated.

For example, when the total power amount is not less than one third andless than two thirds of the total rated power amount output by allconverters 15, 25, 35, converter control unit 300 deactivates converter35 by outputting a number selection command signal “ON” to each ofconverter operation command circuits 301, 302 for converters 15, 25 andoutputting a number selection command signal “OFF” to converteroperation command circuit 303 for converter 35. In addition, when thetotal power amount is less than one third of the total rated poweramount output by all converters 15, 25, 35, converter control unit 300deactivates converters 25, 35 by outputting a number selection commandsignal “ON” to converter operation command circuit 301 for converter 15and outputting a number selection command signal “OFF” to each ofconverter operation command circuits 302, 303 for converters 25, 35.

It is noted that, when the total power amount becomes larger than thetotal rated power amount output by activated converters 15, 25, 35 dueto an increase in the load, converter control unit 300 can stably supplypower to the load by activating the deactivated converter.

Embodiment 2

An uninterruptible power supply system in accordance with Embodiment 2of the present invention includes AC input unit 1, input/outputswitching unit 2, control unit 3, DC branch unit 4, storage battery 5,output unit 6, AC output units 7, 8, and uninterruptible power supplydevice 10, 20, 30, as with uninterruptible power supply system 100 inaccordance with Embodiment 1. Thus, concerning the uninterruptible powersupply system in accordance with Embodiment 2 of the present invention,components identical to those of uninterruptible power supply system 100in accordance with Embodiment 1 will be designated by the same referencenumerals, and a detailed description thereof will not be repeated.

Next, an operation of the uninterruptible power supply system inaccordance with Embodiment 2 of the present invention will be described.FIG. 5 is a flowchart for illustrating the operation of theuninterruptible power supply system in accordance with Embodiment 2 ofthe present invention. First, converter control unit 300 activates apredetermined number of converters 15, 25, 35 of uninterruptible powersupply devices 10, 20, 30 (step S51). Converter control unit 300 setsthe predetermined number of converters to be activated to half of thenumber of converters of the uninterruptible power supply devices. Forexample, when the number of converters of the uninterruptible powersupply devices is four, converter control unit 300 sets thepredetermined number of converters to be activated to two. It is notedthat, when the number of converters of the uninterruptible power supplydevices is an odd number, converter control unit 300 sets a numbercalculated by rounding up the predetermined number of converters to beactivated, as a predetermined number.

Specifically, when the number of converters 15, 25, 35 ofuninterruptible power supply devices 10, 20, 30 is three, convertercontrol unit 300 outputs number selection command signals for selectingtwo converters 15, 25 to converter operation command circuits 301, 302for converters 15, 25, and signal generation circuits 307, 308 outputinstruction signals for activating converters 15, 25 to converteroperation command circuits 301, 302.

Converter operation command circuits 301, 302 output command signals forcommanding activation to converters 15, 25, based on the numberselection command signals output by converter control unit 300 and theinstruction signals output by signal generation circuits 307, 308.Converters 15, 25 are activated based on the command signals output byconverter operation command circuits 301, 302.

Subsequently, converter control unit 300 compares a total power amountoutput by converters 15, 25 for supplying a power amount required forthe load, with a first total rated power amount output by activatedconverters 15, 25, and determines whether or not the total power amountis not less than the first total rated power amount (step S52).

When converter control unit 300 determines that the total power amountis not less than the first total rated power amount (YES in step S52),converter control unit 300 activates deactivated converter 35 (stepS53). For example, when the rated power amount output by each converter15, 25, 35 is 1 kWh, converter control unit 300 determines that, if thetotal power amount is 2.8 kWh, it is not less than the total rated poweramount output by activated converters 15, 25 (1 kWh×2 units=2 kWh).

By converter control unit 300 activating converter 35, the total poweramount (2.8 kWh) can be smaller than the total rated power amount outputby converters 15, 25, 35 (1 kWh×3 units=3 kWh), and thus an overloadoperation can be avoided. Converter control unit 300 activates converter35 by outputting a number selection command signal “ON” to converteroperation command circuit 303 for converter 35.

It is noted that, after converter control unit 300 activates deactivatedconverter 35 (step S53), converter control unit 300 returns theprocessing to step S52 to monitor a change in the total power amount.

When converter control unit 300 determines that the total power amountis smaller than the first total rated power amount (NO in step S52),converter control unit 300 compares the total power amount with a secondtotal rated power amount output, when at least one of activatedconverters 15, 25 is deactivated, by the remaining converter, anddetermines whether or not the total power amount is smaller than thesecond total rated power amount (step S54).

When converter control unit 300 determines that the total power amountis smaller than the second total rated power amount (YES in step S54),converter control unit 300 deactivates at least one of activatedconverters 15, 25 (step S55). For example, when the rated power amountof each converter 15, 25, 35 is 1 kWh, converter control unit 300determines that, if the total power amount is 0.8 kWh, it is smallerthan the total rated power amount output, when at least one of activatedconverters 15, 25 is deactivated, by the remaining converter (1 kWh×1unit=1 kWh).

That is, if the total power amount is 0.8 kWh, even when convertercontrol unit 300 deactivates one converter 25, the total power amount issmaller than the total rated power amount output by activated converter15 (1 kWh×1 unit=1 kWh). Thus, converter control unit 300 deactivatesconverter 25 by outputting a number selection command signal “OFF” toconverter operation command circuit 302 for converter 25.

When converter control unit 300 determines that the total power amountis not less than the second total rated power amount (NO in step S54),converter control unit 300 returns the processing to step S52 to monitora change in the total power amount. Further, after converter controlunit 300 deactivates at least one of activated converters 15, 25 (stepS55), converter control unit 300 returns the processing to step S52 tomonitor a change in the total power amount.

It is noted that, when two converters 15, 25 are activated as describedabove, converter control unit 300 deactivates one of activatedconverters 15, 25, and when three or more converters are activated,converter control unit 300 may deactivate at least one of the activatedconverters.

As described above, according to the uninterruptible power supply systemin accordance with Embodiment 2 of the present invention, in the casewhere the power amount required for the load is smaller than the totalrated power amount output by activated converters 15, 25 (i.e., in thecase of a low load), converter control unit 300 deactivates at least oneof activated converters 15, 25. Therefore, efficiency of utilizing theplurality of uninterruptible power supply devices 10, 20, 30 connectedin parallel can be enhanced. Further, in the uninterruptible powersupply system in accordance with Embodiment 2 of the present invention,in the case where the power amount required for the load is larger thanthe total rated power amount output by activated converters 15, 25(i.e., in the case of an overload), deactivated converter 35 isactivated. Therefore, an overload operation can be avoided, and lives ofuninterruptible power supply devices 10, 20, 30, storage battery 5, andthe like can be increased.

Embodiment 3

An uninterruptible power supply system in accordance with Embodiment 3of the present invention includes AC input unit 1, input/outputswitching unit 2, control unit 3, DC branch unit 4, storage battery 5,output unit 6, AC output units 7, 8, and uninterruptible power supplydevice 10, 20, 30, as with uninterruptible power supply system 100 inaccordance with Embodiment 1. Thus, concerning the uninterruptible powersupply system in accordance with Embodiment 3 of the present invention,components identical to those of uninterruptible power supply system 100in accordance with Embodiment 1 will be designated by the same referencenumerals, and a detailed description thereof will not be repeated.

Next, an operation of the uninterruptible power supply system inaccordance with Embodiment 3 of the present invention will be described.FIG. 6 is a flowchart for illustrating the operation of theuninterruptible power supply system in accordance with Embodiment 3 ofthe present invention. First, converter control unit 300 activates oneof converters 15, 25, 35 of uninterruptible power supply devices 10, 20,30 (step S61).

Specifically, when the number of converters 15, 25, 35 ofuninterruptible power supply devices 10, 20, 30 is three, convertercontrol unit 300 outputs a number selection command signal for selectingone converter 15 to converter operation command circuit 301 forconverter 15, and signal generation circuit 307 outputs an instructionsignal for activating converter 15 to converter operation commandcircuit 301.

Converter operation command circuit 301 outputs a command signal forcommanding activation to converter 15, based on the number selectioncommand signal output by converter control unit 300 and the instructionsignal output by signal generation circuit 307. Converter 15 isactivated based on the command signal output by converter operationcommand circuit 301.

Subsequently, converter control unit 300 compares a total power amountoutput by converter 15 for supplying a power amount required for theload, with a first total rated power amount output by activatedconverter 15, and determines whether or not the total power amount isnot less than the first total rated power amount (step S62). Whenconverter control unit 300 determines that the total power amount is notless than the first total rated power amount (YES in step S62),converter control unit 300 activates one converter 25 of deactivatedconverters 25, 35 (step S63). For example, when the rated power amountoutput by each converter 15, 25, 35 is 1 kWh, converter control unit 300determines that, if the total power amount is 1.8 kWh, it is not lessthan the total rated power amount output by activated converter 15 (1kWh×1 unit=1 kWh).

By converter control unit 300 activating converter 25, the total poweramount (1.8 kWh) can be smaller than the total rated power amount outputby converters 15, 25 (1 kWh×2 units=2 kWh), and thus an overloadoperation can be avoided. Converter control unit 300 activates converter25 by outputting a number selection command signal “ON” to converteroperation command circuit 302 for converter 25.

It is noted that, after converter control unit 300 activates deactivatedconverter 25 (step S63), converter control unit 300 returns theprocessing to step S62 to monitor a change in the total power amount.

When converter control unit 300 determines that the total power amountis smaller than the first total rated power amount (NO in step S62),converter control unit 300 compares the total power amount with a secondtotal rated power amount output, when one of the activated converters isdeactivated, by the remaining converter, and determines whether or notthe total power amount is smaller than the second total rated poweramount (step S64). It is noted that the processing in step S64 isperformed only when two or more converters are activated. Thus, theprocessing in step S64 is skipped when one converter is activated.Hereinafter, a description will be given of a case where two converters15, 25 are activated.

When converter control unit 300 determines that the total power amountis smaller than the second total rated power amount (YES in step S64),converter control unit 300 deactivates one of activated converters 15,25 (step S65). For example, when the rated power amount of eachconverter 15, 25, 35 is 1 kWh, converter control unit 300 determinesthat, if the total power amount is 0.8 kWh, it is smaller than the totalrated power amount output, when one of activated converters 15, 25 isdeactivated, by the remaining converter (1 kWh×1 unit=1 kWh).

That is, if the total power amount is 0.8 kWh, even when convertercontrol unit 300 deactivates one converter 25, the total power amount issmaller than the total rated power amount output by activated converter15 (1 kWh×1 unit=1 kWh). Thus, converter control unit 300 deactivatesconverter 25 by outputting a number selection command signal “OFF” toconverter operation command circuit 302 for converter 25.

When converter control unit 300 determines that the total power amountis not less than the second total rated power amount (NO in step S64),converter control unit 300 returns the processing to step S62 to monitora change in the total power amount. Further, after converter controlunit 300 deactivates one of activated converters 15, 25 (step S65),converter control unit 300 returns the processing to step S62 to monitora change in the total power amount.

As described above, according to the uninterruptible power supply systemin accordance with Embodiment 3 of the present invention, in the casewhere the power amount required for the load is smaller than the totalrated power amount output by activated converters 15, 25 (i.e., in thecase of a low load), converter control unit 300 deactivates one ofactivated converters 15, 25. Therefore, efficiency of utilizing theplurality of uninterruptible power supply devices 10, 20, 30 connectedin parallel can be enhanced. Further, in the uninterruptible powersupply system in accordance with Embodiment 3 of the present invention,in the case where the power amount required for the load is larger thanthe total rated power amount output by activated converter 15 (i.e., inthe case of an overload), one of deactivated converters 25, 35 isactivated. Therefore, an overload operation can be avoided, and lives ofuninterruptible power supply devices 10, 20, 30, storage battery 5, andthe like can be increased.

It is noted that, since the converters are activated or deactivated oneby one in the uninterruptible power supply system in accordance withEmbodiment 3 of the present invention, converters 15, 25, 35(uninterruptible power supply devices 10, 20, 30) are not activatedwastefully, and thus power consumption can be further reduced.

It should be understood that the embodiments disclosed herein areillustrative and non-restrictive in every respect. The scope of thepresent invention is defined by the scope of the claims, rather than thedescription above, and is intended to include any modifications withinthe scope and meaning equivalent to the scope of the claims.

REFERENCE SIGNS LIST

1: AC input unit; 2: switching unit; 3: control unit; 4: DC branch unit;5: storage battery; 6: output unit; 7, 8: AC output unit; 10, 20, 30:uninterruptible power supply device; 11, 21, 31, 44 to 48, 71: breaker;12, 18, 22, 28, 32, 38, 72: transformer; 13, 23, 33, 41 to 43, 61 to 63,75: contactor; 14, 24, 34: AC reactor; 15, 25, 35: converter; 16, 26,36: electrolytic capacitor; 17, 27, 37: inverter; 19, 29, 39: capacitor;51, 52: battery; 73, 76 to 79: switch; 74: thyristor switch; 100:uninterruptible power supply system; 300: converter control unit; 301 to303: converter operation command circuit; 304 to 306: inverter operationcommand circuit; 307 to 309: signal generation circuit.

1. An uninterruptible power supply system, comprising: a plurality ofuninterruptible power supply devices which are connected in parallelwith respect to a load, and switch between power supplies supplyingpower to said load depending on states of said power supplies; a controlunit controlling an operation of switching between said power suppliesby said uninterruptible power supply devices; and a storage batteryconnected to the plurality of said uninterruptible power supply devicesin common, said uninterruptible power supply device having a conversionunit converting AC power of an AC power supply into DC power, aswitching unit switching between the DC power converted by saidconversion unit and DC power input from said storage battery, and aninversion unit inverting the DC power converted by said conversion unitor the DC power input from said storage battery into AC power andsupplying the power to said load, said control unit deactivating saidconversion unit which does not contribute to supplying a power amountrequired for said load, of a plurality of said conversion units.
 2. Theuninterruptible power supply system according to claim 1, wherein saidcontrol unit activates all of said conversion units, and thereaftercompares a total power amount output by the plurality of said conversionunits for supplying the power amount required for said load, with atotal rated power amount output, when at least one of the plurality ofsaid activated conversion units is deactivated, by remaining saidconversion units, and when said total power amount is smaller than saidtotal rated power amount, said control unit deactivates at least one ofthe plurality of said activated conversion units.
 3. The uninterruptiblepower supply system according to claim 1, wherein said control unitactivates a predetermined number of said conversion units, thepredetermined number being not less than two, and thereafter compares atotal power amount output by the plurality of said conversion units forsupplying the power amount required for said load, with a first totalrated power amount output by the plurality of said activated conversionunits, when said total power amount is not less than said first totalrated power amount, said control unit activates said deactivatedconversion unit until said total power amount becomes smaller than saidfirst total rated power amount, when said total power amount is smallerthan said first total rated power amount, said control unit comparessaid total power amount with a second total rated power amount output,when at least one of the plurality of said activated conversion units isdeactivated, by remaining said conversion unit, and when said totalpower amount is smaller than said second total rated power amount, saidcontrol unit deactivates at least one of the plurality of said activatedconversion units.
 4. The uninterruptible power supply system accordingto claim 3, wherein said control unit sets said predetermined number ofsaid conversion units to be activated to half of a number of saidconversion units of the plurality of said uninterruptible power supplydevices.
 5. The uninterruptible power supply system according to claim1, wherein said control unit activates one of said conversion units, andthereafter compares a total power amount output by the plurality of saidconversion units for supplying the power amount required for said load,with a first total rated power amount output by the plurality of saidactivated conversion units, when said total power amount is not lessthan said first total rated power amount, said control unit activatessaid deactivated conversion units one by one until said total poweramount becomes smaller than said first total rated power amount, whensaid total power amount is smaller than said first total rated poweramount, said control unit compares said total power amount with a secondtotal rated power amount output, when one of the plurality of saidactivated conversion units is deactivated, by remaining said conversionunit, and when said total power amount is smaller than said second totalrated power amount, said control unit deactivates one of the pluralityof said activated conversion units.